Polarization and wavelength stable superfluorescent sources

ABSTRACT

The instability of the mean wavelength of a superfluorescent fiber source (SFS) is reduced by randomizing the polarization of light from a pump source or by using polarization maintaining components. In one embodiment, the polarization of a pump source is made more random, leading to greater stability of the mean wavelength of the SFS, with an output mean wavelength that is stable to better than 3 ppm for full rotation of the pump polarization state. In another embodiment, the polarization of optical radiation throughout the device is kept substantially constant by using polarization maintaining fiber and components, thereby leading to enhanced mean wavelength stability of the SFS.

The present application claims the benefit of the following provisionalapplications, all of which are hereby incorporated by reference herein:Application No. 60/106,532 filed Oct. 31, 1998, Application No.60/106,709 filed Nov. 2, 1998, Application No. 60/113,220 filed Dec. 22,1998, and Application No. 60/128,641 filed Apr. 9, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fiber amplified spontaneous emission(ASE) light sources, and more particularly, to superfluorescent fibersources that have a stable mean wavelength with respect to changes inpump polarization.

2. Description of the Related Art

Fiber ASE light sources are well known in the art. ASE sources have beenadvantageously used to provide wideband (e.g., on the order of 10 to 30nanometers), spatially coherent light for multiple applications. Forexample, ASE sources have been used to provide laser light as an inputto a fiberoptic gyroscope. For a description of an exemplarysuperfluorescent fiber source, see an article entitled “Amplification ofSpontaneous Emission in Erbium-Doped Single-Mode Fibers” by EmmanuelDesurvire and J. R. Simpson, published by IEEE, in “Journal of LightwaveTechnology,” Vol. 7, No. May 5, 1989.

An ASE light source typically comprises a length of single-mode fiber,with a portion of its cross-section (typically the core) doped with anionic, trivalent rare-earth element. For example, neodymium (Nd³⁺) anderbium (Er³⁺) are rare-earth elements that may be used to dope the coreof a single-mode fiber so that it acts as a laser medium.

The fiber receives a pump input signal at one end. The pump signal istypically a laser signal having a relatively narrow spectrum centeredaround a wavelength λ_(p). The ions within the fiber core absorb theinput laser radiation at wavelength λ_(p) so that electrons in theground state of these ions are excited to a higher energy state of theions. When a sufficient pump power is input into the end of the fiber, apopulation inversion is created (i.e., more electrons within the ionsare in the excited state than are in the lower laser state), and asignificant amount of fluorescence is generated along the length of thefiber. As is well known, the fluorescence (i.e., the emission of photonsat a different wavelength λ_(s)) is due to the spontaneous return ofelectrons from the excited state to the lower laser state so that aphoton at a wavelength λ_(s) is emitted during the transition from theexcited state to the ground state. These photons are amplified by thegain as they travel down the fiber, leading to amplified spontaneousemission (ASE). The light which is emitted at the wavelength λ_(s) fromthe fiber is highly directional light, as in conventional laser light.However, one main characteristic of this emission which makes itdifferent from that of a traditional laser (i.e., one which incorporatesan optical resonator) is that the spectral content of the light emittedfrom the superfluorescent fiber source is generally very broad(typically several tens of nanometers). This principle is well known inlaser physics, and has been studied experimentally and theoretically insilica-based fibers doped with erbium, neodymium, or other rare earths,for several years.

Light emitted from ASE fiber sources has multiple applications. Forexample, in one application, the output of the ASE source is fed into afiberoptic gyroscope. For reasons that are well understood by thoseskilled in the art, the fiberoptic gyroscope should be operated with abroadband source which has a highly stable mean wavelength. Of theseveral types of broadband sources known to exist, superfluorescentfiber sources, in particular, made with erbium-doped fiber, have beenthus far the only optical sources which meet the stringent requirementsfor inertial navigation grade fiberoptic gyroscopes. The broad bandwidthof light produced by erbium-doped fiber sources, together with the lowpump power requirements and excellent mean wavelength stability oferbium-doped fiber sources, are the primary reasons for use of suchsources with fiberoptic gyroscopes.

In an erbium-doped fiber, the emission of a superfluorescent fibersource is bi-directional. That is, the light which is emitted by thereturn of electrons to the ground state in the erbium ions is typicallyemitted out of both ends of the fiber. As described in U.S. Pat. No.5,185,749, to Kalman, et al., for erbium-doped fibers of sufficientlength, the light propagated in the backward direction (i.e., in thedirection opposite that in which the pump signal propagates) has a veryhigh efficiency. Thus, it is advantageous to implement erbium-dopedsources so that the light emitted from the ASE erbium-doped source isemitted from the pump input end of the fiber (i.e., in the backwardpropagation direction).

An ASE source is generally implemented in one of two configurations. Ina first configuration, called a single-pass ASE source, thesuperfluorescent source output power is emitted in two directions, oneof which is not used. In the second configuration, called a double-passASE source, a reflector is placed at one end of the doped fiber toreflect the superfluorescent source signal so that the superfluorescentsignal is sent a second time through the fiber. Since the fiber exhibitsgain at the superfluorescent signal wavelengths, the ASE signal isfurther amplified. One advantage of the double-pass configuration isthat it produces a stronger signal. A double-pass ASE sourceconfiguration also produces output only at one port (i.e., in onedirection). A disadvantage of such a configuration is that the feedbackoptical signal from the gyroscope must be kept very low in order toprevent lasing (e.g., with use of an optical isolator located betweenthe source and the gyroscope).

For fiberoptic gyroscope applications, one critical measure of sourceperformance is the stability of the source mean wavelength (for example,see U.S. Pat. No. 5,355,216 to Kim, et al.). As is well known in theart, stability of the source mean wavelength leads directly to thestability of the gyroscope scale factor. Precise knowledge of the scalefactor is critical for an accurate measurement of the rotation rate ofthe gyroscope. Presently, superfluorescent fiber sources exist whichhave a mean wavelength stability with respect to pump power, pumpwavelength, temperature, and level of optical feedback down to a fewparts per million each, assuming reasonable stabilization of systemparameters such as pump wavelength, pump power, temperature and opticalfeedback from the gyroscope. However, an overall stability of betterthan one part per million in mean wavelength is desirable for someapplications, in particular, high-grade fiberoptic gyroscopes.

Polarization effects have recently been shown to play a role in theinstability of the mean wavelength of superfluorescent fiber sources(SFS). The polarization dependence of the mean wavelength of an SFSoutput has been predicted through numerical modeling by J. L. Wagener,et al. [see J. L. Wagener, “Erbium doped fiber sources and amplifiersfor optical sensors,” Ph.D. thesis, Applied Physics Department, StanfordUniversity (March 1996); J. L. Wagener, M. J. F. Digonnet, and H. J.Shaw, “A High-Stability Fiber Amplifier Source for the Fiber OpticGyroscope,” J. Lightwave Technol. Vol. 15, 1689-1694 (Sep. 1997); and J.L. Wagener, D. G. Falquier, M. J. F. Digonnet, and H. J. Shaw, “AMueller Matrix Formalism for Modeling Polarization Effects inErbium-Doped Fiber,” J. Lightwave Technol. Vol. 16, 200-206 (February1998), which are hereby incorporated by reference herein]. These studieshave shown that the mean wavelength of the SFS depends slightly on pumppolarization. The reason for this can be explained in physical terms asfollows. The ions of erbium (or another dopant, such as Nd or anotherrare earth) in the fiber host experience an intrinsic anisotropy ofabsorption and emission with respect to polarization. For example, someerbium ions more strongly absorb a given polarization than others, andcorrespondingly, these erbium ions have a preferred polarizationassociated with their emission. This effect gives rise topolarization-dependent gain when the erbium-doped fiber is pumped in theusual manner, i.e., by a highly polarized source such as a laser diode.This in turn can result in orthogonal polarization components of theoutput ASE signal having different mean wavelengths.

SUMMARY OF THE INVENTION

A first embodiment of the invention is a superfluorescent source thatincludes an optical pump source that generates optical radiation that issubstantially unpolarized and an optically active solid state medium(e.g., a solid state laser medium) that is pumped by the substantiallyunpolarized optical radiation. The medium has characteristics selectedto generate superfluorescence having a full width at half maximum (FWHM)of at least 2 nm and a mean wavelength that is stable to within 50 ppmagainst (i.e., even in the presence of) polarization fluctuations in thesuperfluorescent source. In one preferred embodiment, thesuperfluorescence has a mean wavelength that is stable to within 3 ppmin the presence of polarization fluctuations in the superfluorescentsource. In a preferred embodiment, the mean wavelength is stable towithin 50 ppm in the presence of birefringence changes in thesuperfluorescent source. In one preferred embodiment, the meanwavelength is stable to within 50 ppm in the presence of polarizationchanges of the optical radiation from the optical pump source.

Another embodiment of the invention is a superfluorescent source thatincludes an optical pump source that generates optical radiation that issubstantially unpolarized and an optically active solid state medium(e.g., laser medium) that is pumped by the substantially unpolarizedoptical radiation. The medium has characteristics selected to generatesuperfluorescence having a full width at half maximum (FWHM) of at least2 nm and a mean wavelength that is stable to within 50 ppm even in thepresence of polarization changes in the source that range over thePoincaire sphere.

Yet another embodiment is a superfluorescent source that includes anoptical pump source that generates optical radiation that issubstantially unpolarized. The optical pump source includes a pluralityof pumps that generate respective optical outputs, a polarization mixerthat receives the respective optical outputs from the plurality of pumpsand generates optical output (in which the respective optical outputsfrom the plurality of pumps have polarizations selected such thatoptical output from the mixer is substantially unpolarized), and adepolarizer that receives the optical output from the polarizationmixer. The embodiment further includes an optically active solid statemedium (e.g., laser medium) that is pumped by the output from thedepolarizer, in which the medium has characteristics selected togenerate superfluorescence having a full width at half maximum (FWHM) ofat least 2 nm and a mean wavelength that is stable in the presence ofpolarization fluctuations in the superfluorescent source. In a preferredembodiment, the superfluorescent source has a mean wavelength that isstable to within 500 ppm in the presence of polarization fluctuations inthe superfluorescent source. In a more preferred embodiment, the meanwavelength is stable to within 100 ppm in the presence of polarizationfluctuations in the superfluorescent source. In a still more preferredembodiment, the mean wavelength is stable to within 50 ppm in thepresence of polarization fluctuations in the superfluorescent source. Ina most preferred embodiment, the mean wavelength is stable to within 3ppm in the presence of polarization fluctuations in the superfluorescentsource. In one preferred embodiment, the plurality of pumps includes twopumps having respective optical outputs whose polarizations are combinedso that their polarizations are orthogonal to each other. In a preferredembodiment, the mean wavelength is stable to within 500 ppm in thepresence of birefringence changes in the superfluorescent source. In onepreferred embodiment, the mean wavelength is stable to within 500 ppm inthe presence of polarization changes of the optical radiation from theoptical pump source.

Yet another preferred embodiment is a method of generatingsuperfluorescence, which includes providing a plurality of optical pumpshaving respective optical outputs with different polarizations,directing the respective optical outputs through a polarization mixerthat produces optical output (in which the different polarizations areselected so that the optical output from the mixer is substantiallyunpolarized), depolarizing the output from the mixer, injecting thedepolarized output into an optically active solid state medium (e.g.,laser medium), and producing superfluorescence from the medium that hasa mean wavelength that is stable in the presence of polarizationfluctuations in the superfluorescent source. In a preferred embodiment,the wavelength is stable to within 500 ppm in the presence ofpolarization fluctuations in the superfluorescent source. In a preferredembodiment, the plurality of optical pumps includes two pumps havingrespective optical outputs combined so that their polarizations areorthogonal to each other.

Yet another embodiment is a method of generating superfluorescence,comprising providing an optically active medium (e.g., laser medium)having first and second ends, pumping the first end of the medium withoptical output from a first optical pump (in which the output from thefirst optical pump has a first power and a first polarization), andpumping the second end of the medium with optical output from a secondoptical pump (in which the output from the second optical pump has asecond power and a second polarization different from the firstpolarization). The method further includes producing optical output fromthe first end of the medium that comprises a first spectral componenthaving a first mean wavelength and a polarization parallel to the firstpolarization, and a second spectral component having a second meanwavelength and a polarization orthogonal to the first polarization. Themethod also includes selecting the first pump power and the second pumppower so as to substantially reduce the polarization dependent gain thatwould be present if the first power were equal to the second power, sothat the difference between the mean wavelength of the first spectralcomponent and the mean wavelength of the second spectral component issubstantially reduced. In a preferred embodiment, the first polarizationand the second polarization are orthogonal. In one preferred embodiment,the second power is selected to be less than the first power.

Another embodiment of the invention is a device that includes an opticalpump that produces polarized optical output. The source further includesan optically active, solid state medium (e.g., laser medium) thatreceives the polarized optical output, in which the medium hasbirefringence axes that receive equal amounts of pump power to reducepolarization dependent gain effects within the medium. The mediumproduces optical output that has substantially the same mean wavelengthfor all polarization. The device further includes a fiber opticgyroscope that receives the optical output from the medium.

Yet another embodiment of the invention is a method of generatingsuperfluorescent optical output that includes outputting a polarizedoptical signal from a pump source (in which the polarized optical signalhas a polarization axis), inputting the polarized optical signal into anoptically active, solid state medium (e.g., laser medium) that hasbirefringence axes, and orienting the birefringence axis of the solidstate medium at about 45 degrees with respect to the polarization axisof the polarized optical output to reduce polarization dependent gaineffects within the medium such that the solid state medium produces asuperfluorescent optical output that has substantially the same meanwavelength for all polarizations.

Still another embodiment of the invention is a method of generatingsuperfluorescent output from a superfluorescence source that includesproviding an optical pump which generates optical output and directingthe optical output into a polarization mixer which generates a firstoutput signal and a second output signal (in which the two outputsignals having respective intensities and different polarizations). Thefirst output signal is directed into a first end of a optically activesolid state medium (e.g., laser medium), and the second output signal isdirected into a second end of the optically active solid state medium.Optical gain is produced in the solid state medium that is substantiallyindependent of polarization to generate optical output from one end ofthe solid state medium whose mean wavelength is stable even in thepresence of polarization fluctuations in the superfluorescent source. Ina preferred embodiment, the gain that is substantially independent ofpolarization is produced by selecting the intensities of the first andsecond output signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an experimental setup for investigating the variations inthe mean wavelength of a superfluorescent fiber source as thepolarization of the optical pump is varied with a polarizationcontroller located at the output end of the optical pump.

FIG. 2 presents data collected with the setup shown in FIG. 1.

FIG. 3 shows an experimental setup similar to that of FIG. 1, exceptthat a depolarizer is used at the output end of the optical pump.

FIG. 4, comprising FIGS. 4A, 4B, 4C, and 4D, presents data showing thevariations in the mean wavelength of a superfluorescent fiber source aspolarization is varied for the experimental setups of FIGS. 5 and 9B.

FIG. 5 shows an experimental setup for investigating the range of meanwavelengths from a superfluorescent fiber source as the polarization ofthe optical pump is varied, in which a polarization controller has beenplaced at the output end of the optical pump.

FIG. 6 shows typical transmission spectra from a Lyot depolarizer thatcomprise a nearly periodic series of maxima and minima.

FIG. 7 shows the spectral output from a hypothetical light source toillustrate the behavior of the Lyot depolarizer of FIG. 6.

FIG. 8 shows an embodiment having an output whose mean wavelength isstable with respect to external perturbations, in which linearlypolarized pump light is coupled directly into a depolarizer made fromhigh birefringence fiber.

FIG. 9A shows an embodiment in which a second depolarizer, such as aLyot depolarizer, is advantageously added downstream from a WDM coupler.

FIG. 9B shows an experimental setup for investigating the range of meanwavelengths from a superfluorescent fiber source as a function ofpolarization, and in particular, the effect that a wavelength divisionmultiplexing (WDM) coupler may have on that range when the WDM has somepolarization dependence.

FIG. 9C shows an embodiment that comprises a fiber optic gyroscope andup to three depolarizers located at various points in the device.

FIG. 10 shows an experimental setup for assessing the effectiveness ofdepolarizers in reducing variations in the mean wavelength of an SFS dueto environmental perturbations, such as temperature variations, in whichan erbium-doped fiber is placed in a water bath.

FIG. 11 shows experimental results obtained with experimental setup ofFIG. 10, in which the mean wavelength of the source is plotted versustime.

FIG. 12A shows an embodiment for reducing the pump polarizationdependence of the source mean wavelength, in which the superfluorescentsource is pumped with two linearly polarized pump sources withsubstantially the same spectrum and polarization, in a backwardconfiguration.

FIG. 12B shows an embodiment for reducing the pump polarizationdependence of the source mean wavelength which employs two linearlypolarized pump sources in a forward configuration.

FIG. 12C shows an embodiment for reducing the pump polarizationdependence of the source mean wavelength which employs two linearlypolarized pump sources in a double-pass configuration.

FIG. 12D shows an embodiment that is similar to the embodiment of FIG.12C except that a depolarizer has been added.

FIG. 13A shows an embodiment for reducing the pump polarizationdependence of the source mean wavelength, in which the superfluorescentfiber source is bidirectionally pumped.

FIG. 13B presents the results of a simulation corresponding to theembodiment of FIG. 13A, in which the power of the left pump is fixed at30 mW, and the mean wavelength difference between polarizations of theamplified spontaneous emission (ASE) traveling from left to right isplotted as a function of the power of the right pump.

FIG. 13C presents the results of a simulation corresponding to theembodiment of FIG. 13A, in which the power of the left pump is fixed at30 mW, and the mean wavelength difference between polarizations of theamplified spontaneous emission (ASE) traveling from right to left isplotted as a function of the power of the right pump.

FIG. 14A shows an embodiment for producing a stable spectrum and meanwavelength from a superfluorescent fiber source (backward configuration)that utilizes polarization maintaining optical components.

FIG. 14B shows a fiber optic gyroscope embodiment based on thepolarization maintaining design of FIG. 14A.

FIG. 14C shows a fiber optic gyroscope embodiment similar to that ofFIG. 14B, except that a polarizer has been added to the superfluorescentfiber.

FIG. 15 shows a double pass configuration for reducing the pumppolarization dependence of the source mean wavelength, which utilizespolarization maintaining components.

FIG. 16 shows a backward source configuration for reducing the pumppolarization dependence of the source mean wavelength, which utilizespolarization maintaining components and a WDM coupler whosebirefringence axes are aligned at 45 degrees to the polarization of thepump.

FIG. 17 shows a double pass source configuration for reducing the pumppolarization dependence of the source mean wavelength, which utilizespolarization maintaining components and a WDM coupler whosebirefringence axes are aligned at 45 degrees to the polarization of thepump.

FIG. 18 shows another double pass source configuration for reducing thepump polarization dependence of the source mean wavelength, whichutilizes polarization maintaining components and a WDM coupler whosebirefringence axes are aligned at 45 degrees to the polarization of thepump.

FIG. 19 shows a configuration for reducing the pump polarizationdependence of the superfluorescent source mean wavelength which utilizesa single pump source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described below in connection with opticalwaveguides in the form of optical fibers. It should be understood thatother types of optical waveguides can be advantageously substituted forthe optical fibers in many of the embodiments described herein. The term“solid state” as used herein includes optical waveguides such as opticalfibers.

The intrinsic anisotropy of erbium ions in a host such as silica and thehigh degree of polarization of a laser pump source result inpolarization-dependent gain. Although this effect is small, it hassignificant deleterious effects in optical communication systemsutilizing multiple erbium-doped fiber amplifiers, and, for theseapplications, the polarization dependence of the gain should be reduced.Polarization-dependent gain may also play a deleterious role in asuperfluorescent fiber source. The reason is that the mean wavelength ofthe source is affected, through polarization-dependent gain, by the pumppolarization. Typically, the rotation sensing coil within a fiber opticgyroscope is preceded (in the optical path sense) by a polarizer. Thus,if the polarization of the pump entering the erbium-doped fiber driftsas a result of environmental changes (changes in the fiber birefringenceinduced by variations in temperature, or variations in the orientationof the fiber, will change the state of polarization (SOP) of the lightin the erbium-doped fiber), the mean wavelength entering the gyro coilvaries, as does the gyro scale factor. As discussed earlier, sucheffects are highly undesirable in fiber optic gyroscope (FOG)applications, in particular in high-grade gyros, in which an SFS sourcewith mean wavelength variations under one part per million (ppm) isrequired.

It is one object of this invention to reduce the instability of the meanwavelength of a superfluorescent fiber source (SFS). In one embodimentof the invention, the polarization of a pump source of asuperfluorescent fiber source is made more random, leading to greaterstability of the mean wavelength of the SFS. Using one or moredepolarizers at the output of the pump source as well as at otherlocations in the SFS dramatically reduces the SFS output spectrumdependence on polarization, so that polarization drift of the pump, orthe superfluorescence (e.g., due to changes in the birefringence ofoptical components in the SFS), or both, are inconsequential. In anotherembodiment of the invention, the polarization of optical radiationthroughout the device is kept substantially constant by usingpolarization-maintaining fiber and components, thereby leading toenhanced mean wavelength stability of the SFS. Usingpolarization-maintaining components does not produce fully unpolarizedoutput, but this is unimportant to the source stability because thedependence on pump polarization is removed by eliminating polarizationdrifts altogether.

Experimental data related to a first embodiment of the invention arecollected using the setup shown in FIG. 1. An optical pump source 100such as a laser diode has an output pigtail 104 spliced to apolarization controller 106 that is spliced to an optical couplingelement 110 such as a wavelength division multiplexing (WDM) fibercoupler. The optical pump source 100 may alternatively comprise one ormore light emitting diodes (LEDs). The WDM coupler 110 is in turnspliced to a doped optical fiber 118 capable of producing optical outputin the form of superfluorescent light. The optical fiber 118 ispreferably an Er-doped fiber of silicon dioxide. (In the embodimentsdisclosed herein, other rare earths such as neodyrnium may also beused.) For the results reported here, the fiber is 15 meters in lengthand its small-signal absorption is 12 dB/m at 1480 nm and 27 dB/m at1530 nm. The WDM coupler 110 is further spliced to an input pigtail 120of an isolator 124, which has an output pigtail 128 that directs theoptical output from the fiber 118 to a combination depolarizer/opticalspectrum analyzer (DOSA) 136. The isolator 124 prevents unwanted opticalfeedback from the DOSA 136 from affecting the spectrum of thesuperfluorescence emitted by the doped fiber 118.

For the purpose of this measurement, it is important that the opticalspectrum analyzer (OSA) exhibit no polarization dependence, i.e., thatif different polarizations of the same spectrum of light are fed intothe OSA, the spectra read and supplied by the OSA are the same. However,the OSA used for this measurement exhibited some polarizationdependence, i.e., it acted as a polarization-independent OSA preceded bya frequency-dependent partial polarizer. To eliminate this undesirableproperty, a fiber depolarizer was placed in front of the OSA. The fiberdepolarizer, as described below, was made of two lengths ofhigh-birefringence fiber spliced together at a 45° angle. The firstfiber length was 1.5 meters, and the second fiber length was 3 meters.When the polarization of spectrally broad light fed into the DOSA 136 ischanged, the spectrum read by the DOSA remains unchanged, so that theDOSA acts as a polarization-independent instrument.

As indicated in FIG. 1, the superfluorescent output signal emerging fromthe Er-doped fiber 118 passes through the isolator 124 before beingdetected by the DOSA 136. Digitized output from the DOSA 136 can berecorded and displayed by a computer 140 which calculates spectrumstatistics, and in particular, calculates the mean wavelength of thespectrum. The data acquisition rate was selected such that this setupscans and displays a new spectrum, then calculates and displays its meanwavelength, once every few seconds. The setup of FIG. 1 thus comprises abackward SFS pumping arrangement in which the mean wavelength of thespectral output can be monitored over time.

FIG. 2 shows data collected with the setup of FIG. 1, in which the SFSmean wavelength is plotted against scan number. Before taking each datapoint in the plot, the polarization controller 106 was adjusted so thata different polarization of the pump radiation entered the doped fiber118. The polarization controller 106 was thus adjusted repeatedly toprovide substantially all possible pump polarization states (or allpoints on the Poincaire sphere). However, the polarization controller106 was not necessarily varied in a regular, systematic way by, forexample, changing the polarization by a fixed amount from scan to scan.Thus, the mean wavelengths are not expected to exhibit any trend withscan number.

The observed range in mean wavelength relative variations (i.e., Δλ/λ)in FIG. 2 is approximately 110 ppm, which is much larger than the meanwavelength stability required for a high-grade fiber optic gyroscope,which is preferably <1 ppm. Thus, even if the state of polarization ofthe light entering the fiber 118 were to change by only a relativelysmall fraction, the SFS mean wavelength would still vary by an amountsubstantially greater than 1 ppm. Such changes can easily occur overlong time periods (an hour or more), e.g., changes in the environmentcan alter the birefringence of any portion of the optical link betweenthe pump source 100 and the far end of the erbium-doped fiber 118, suchas the output pigtail 104, the WDM coupler 110, and the erbium-dopedfiber 118. Accordingly, environmental factors can produce a slow driftin the SFS mean wavelength, making the apparatus of FIG. 1 unsuitablefor use in systems requiring a highly stable mean wavelength.

In accordance with a first embodiment of the present invention, thepolarization controller 106 shown in FIG. 1 is replaced by a depolarizer300, as illustrated in FIG. 3. As is well known in the art, adepolarizer is a device that randomizes the state of polarization of alight source by, for example, scrambling its polarization rapidly overtime. Alternatively, a depolarizer may give different states ofpolarization to different frequency components of an optical source, sothat the light is no longer characterized by a single state ofpolarization, but rather, the polarization is a strong function ofwavelength.

The WDM coupler 110 of the embodiment shown in FIG. 3 advantageously hasa coupling ratio and a loss that are independent of polarization, sothat depolarized pump light from the depolarizer 300 is stilldepolarized where it enters the erbium-doped fiber 118. As a result,polarization-dependent gain within the fiber 118 is greatly reduced, andthe output of the superfluorescent optical fiber 118 (at the pointindicated by the arrow 310) is unpolarized. In other words, if anadditional polarizer (not shown) were inserted between the isolator 124and the OSA 136, the output spectrum at the OSA would be substantiallyidentical for all rotational orientations of this additional polarizer,assuming that the transmission of the isolator 124 from left to right isindependent of polarization.

The depolarizer 300 may comprise a number of depolarizers known in theart, including, but not limited to, a polarization scrambler (e.g., afast PZT modulator) and a Lyot depolarizer. Other kinds of depolarizersare discussed in the literature. (See, for example, A. D. Kersey, and A.Dandridge, “Monomode fibre polarization scrambler,” Electron. Lett. Vol.23, 634-636, June 1987; and K. Takada, K. Okamota, and J. Noda, “Newfiber-optic depolarizer,” J. Lightwave Technol. Vol. 4, 213-219,February 1986, which discusses a Mach-Zehnder interferometer with adelay line). In its simplest form, a PZT-based depolarizer may comprisea PZT ring about which is wound an optical fiber. A voltage applied tothe ceramic ring is rapidly modulated, causing the size of the ring tovary, so that when the polarization of light is properly aligned withthe PZT ring, the polarization of the light propagating through theoptical fiber varies with time. A PZT depolarizer suitable fordepolarizing an arbitrary input polarization may advantageously comprisetwo PZT rings that are orthogonal to each other.

A Lyot depolarizer was used for the depolarizer 300 of FIG. 3 to collectthe experimental results presented below (FIG. 4). A Lyot fiberdepolarizer generally comprises two lengths of highly birefringent fiberspliced together (for the experimental results reported herein, the Lyotdepolarizer comprised 1 meter and 1.5 meter lengths), with theirbirefringence axes oriented 45° with respect to each other. (See, forexample, K. Böhm, K. Petermann, and E. Weidel, “Performance of Lyotdepolarizers with birefringent single-mode fibers,” J. LightwaveTechnol. Vol. 1, 71-74, March 1983.) A Lyot fiber depolarizer transformspolarized light into light with a state of polarization that dependsstrongly on wavelength. For example, a 30 nanometer (nm) broad spectrumcharacterized by a single polarization may be converted by a Lyotpolarizer into, say, ten adjacent spectral intervals of 3 nm width each,with adjacent intervals having orthogonal polarizations.

Thus, if the spectrum of the light is broad enough, and if the lengthsof the fibers in the Lyot depolarizer are long enough, the light outputby the depolarizer will carry the same power in any two orthogonallinear polarizations, and the mean wavelength of these two polarizationswill be identical.

For the results presented in FIG. 4 below, the pump source 100 washighly polarized, having an extinction ratio (the power of the moreintense polarization divided by the power of the less intensepolarization, in which the two polarizations are orthogonal) greaterthan 30 dB. After passing through the depolarizer 300, the pump lighthad an extinction ratio of around 3 dB or better, as described below.Experiments show that even a depolarizer that reduces the extinctionratio of the pump source to around 3 dB is sufficient to significantlyimprove the mean wavelength stability of the optical output of the fiber118.

The foregoing is illustrated in FIGS. 4A and 4B. FIG. 4A, labeled“Baseline,” represents a series of mean wavelength measurements usingthe embodiment of FIG. 5, which is similar to the embodiment of FIG. 3,except that a first polarization controller 106 (designated here as PC₁)has been added between the pump source 100 and the depolarizer 300.After passing through the depolarizer 300, the pump light had anextinction ratio that depends on the state of polarization of the lightat the input of the depolarizer. For some input polarizations, i.e., forsome orientations of the polarization controller PC₁, the light at theoutput of the depolarizer 300 is strongly depolarized. However, forother orientations of polarization controller PC₁, after passing throughthe depolarizer 300, the pump light had an extinction ratio of onlyabout 3 dB. No parameters are varied during these tests, and thefluctuation in the measured mean wavelength represents the system noise.The data of FIG. 4B are also collected with the apparatus of FIG. 5,using the same methodology used to generate the results of FIG. 2. Inparticular, the orientation of the polarization controller is variedbetween scans to determine the extent of the variation of meanwavelength with changes in the pump polarization incident on thedepolarizer 300. In FIG. 4B, the variation in the SFS mean wavelength isonly about 3 ppm, which is a considerable improvement over the 110 ppmvariations (see, for example, FIG. 2) observed using the same opticalpump source 100 in the absence of a depolarizer 300. Further, this 3 ppmlevel appears to be no greater than the noise inherent in theexperimental apparatus (see, for example, FIG. 4A), suggesting that theSFS stability could in fact be significantly better than 3 ppm. Thus,one conclusion to be drawn from FIGS. 4A and 4B is that the depolarizer300 makes the optical output from the fiber 118 largely insensitive toperturbations in the pump polarization and to changes in thebirefringence of the fiber pigtail 104. In preferred embodiments of theinvention described herein, optical output is generated in which themean wavelength is stable to within 100 ppm, and more preferably towithin 50 ppm, and still more preferably to within 3 ppm.

The reason why the pump light is still about 3-dB polarized afterpassing through the Lyot depolarizer is now considered. For any givenlinear polarization input into a Lyot depolarizer, and for any givenlinear polarization at the output of the depolarizer, the transmissionspectrum f₁ of the depolarizer comprises a nearly periodic series ofmaxima and minima, as illustrated by curve (a) in FIG. 6. As illustratedby curve (b) in FIG. 6, the transmission spectrum f₂ of the depolarizerfor the orthogonal output polarization also comprises an approximatelyperiodic series of maxima and minima, which are out of sequence withthose of the transmission spectrum f₁. The spacing between maxima (orbetween minima) of both spectra decreases as the lengths of birefringentfiber used in the depolarizer are increased. If the (linear)polarization of the input light is rotated, the transmission spectra f₁and f₂ will simply shift with wavelength, but their respective shapeswill remain unchanged. As illustrated in FIG. 7, the broadband lightemitted by a semiconductor source, such as those commonly used as pumpsources for SFS sources, often exhibits a series of modes. If pump lighthaving the spectrum of FIG. 7 is launched into a Lyot depolarizer andthat light has a polarization that matches that of curve (a) of FIG. 6,then the light will not be well depolarized by the depolarizer, becausemost of the light's frequency components will be transmitted by thedepolarizer without altering its polarization. This is why partiallypolarized light is observed for certain positions of the polarizationcontroller PC₁ at the output of the depolarizer 300. For certain inputpolarizations the pump light accidentally matched the transmissionspectrum of the depolarizer reasonably well, and light was poorlydepolarized. For other positions of the polarization controller PC₁,however, the light output by the depblarizer 300 was much more stronglydepolarized.

A possible remedy to this problem is to select an input polarization forthe light that yields strongly depolarized output light. However, if thelight is fed into the Lyot depolarizer through a standard,low-birefringence fiber, the state of polarization of the light enteringthe depolarizer will vary with environmentally induced changes in thebirefringence of the low-birefringence fiber. A preferred solution is toselect the lengths of the two fibers forming the depolarizer such thatfor the given spectrum of the pump source, and for all possible inputpolarizations, the transmission spectrum of the depolarizer is verydifferent from the light spectrum.

Another way of solving the aforementioned difficulty uses a depolarizerand couples linearly polarized pump light directly into the depolarizer,with the polarization of the light aligned at 45° to the axes of thedepolarizer's high-birefringence fiber, as shown in FIG. 8. The firstbenefit of this configuration is that since the polarization of thelight incident on the depolarizer is fixed, a second length ofhigh-birefringence fiber at 45° to the first length is not needed. Sucha depolarizer 1200 (FIG. 8) is advantageously made of a single length ofhigh-birefringence fiber, which eliminates the need for a difficult andslightly lossy 45° splice between high-birefringence fiber, therebyreducing the cost of the depolarizer. The depolarizer 1200 may beadvantageously butt-coupled to the pump 100. The second benefit is thatthe state of polarization of the light entering the depolarizer 1200 isstable against external perturbations. Consequently, the degree to whichthe pump light is depolarized is invariant in time, and the stability ofthe mean wavelength of the SFS pumped by this depolarized pump light isincreased.

In general, the optical properties of the WDM coupler 110 and theoptical isolator 124 exhibit some dependence on polarization, which willaffect the spectrum, and possibly the mean wavelength, of the broadbandlight returning from the doped fiber 118. As a first case of interest, a“type I” polarization dependence is considered, namely a WDM couplerwith a coupling ratio that has a wavelength-dependent polarizationdependence. In this case, the coupling ratio is different for the twoorthogonal states of polarization, and the ratio R of the couplingratios at two orthogonal polarizations is not the same at allfrequencies across the bandwidth of the ASE light. For example, at 1553nm the coupler couples 98% for a given linear polarization and 100% forthe orthogonal polarization (a ratio R=0.98), while at 1555 nm, thecoupler couples 95% for the same first polarization and 99% for the sameorthogonal polarization (a ratio R=0.95/0.99≈0.96). If fully unpolarizedASE light from the doped fiber 118 is launched into such a coupler, thelight transmitted by the coupler (i.e., at a point between the coupler110 and the isolator 124) will have a spectral shape that is differentfor the two polarizations, because the coupler has applied adifferential filter function to the two polarizations. Therefore, thespectrum of the light transmitted by the coupler will have a differentmean wavelength for the two polarizations. When such light is used asinput into a gyroscope coil, for example, and if the birefringence ofany portion of the fiber between the coupler and the gyroscope coilvaries (e.g., due to external perturbations), the spectrum launched intothe gyroscope coil, and thus the mean wavelength of this spectrum, willvary, which is undesirable.

For example, the WDM coupler 110 used to generate the data presentedhere (manufactured by Gould Fiber Optics of Millersville, Md.) exhibitstype I behavior. When substantially unpolarized broadband light in the1.55 μm region is launched into it, the difference in the meanwavelength of orthogonal polarizations of the light transmitted by thecoupler was measured to be 175 ppm. In order to reduce the effects of acoupler with a type I polarization dependence, a second depolarizer 302,such as a Lyot depolarizer, is advantageously added downstream from theWDM coupler, as illustrated in the embodiment of FIG. 9A.

The same general concern arises regarding the isolator 124. If thetransmission of the isolator 124 depends on polarization, and thispolarization dependence is a function of frequency, then the isolator124 will modify the spectral content of two orthogonally polarized lightsignals differently, which is undesirable for the reasons mentionedabove. Tests indicated that the polarization dependence of thetransmission of the isolator 124 was less pronounced than thepolarization dependence of the coupling ratio of the coupler. Thepolarization dependence of the isolator 124 is reduced by placing thesecond depolarizer 302 downstream from the isolator 124, as illustratedin FIG. 9A.

Next, a WDM coupler with a “type II” polarization dependence isconsidered, namely a WDM coupler having a coupling ratio whosewavelength-dependence is independent of polarization. In other words,the coupling ratio is different for the two orthogonal states ofpolarization, but the ratio R of the coupling ratios for orthogonalpolarizations is the same at all frequencies across the bandwidth of theASE light. If fully unpolarized ASE light from the doped fiber 118 islaunched into such a coupler, the light transmitted by the coupler(i.e., at a point between the coupler 110 and the isolator 124) willhave a spectrum that is identical for each of the two aforementionedorthogonal polarizations, except that one polarization will carry morepower than the other. When such light is used as input into a gyroscopecoil, and the birefringence of any portion of the fiber between thecoupler 110 and the gyro coil varies, the power launched into thegyroscope coil will vary, though not by a large amount if R is not toofar from unity, but the mean wavelength of the light launched into thegyro will be polarization independent. Consequently, a seconddepolarizer 302 is not required to correct this type of polarizationdependence. Similarly, if the transmission of the isolator 124 alsoexhibits a type II polarization dependence, the isolator 124 will notaffect the mean wavelength of the light passing through it, and a seconddepolarizer 302 in FIG. 9A is not required. In one preferred embodiment,the SFS uses a WDM and an isolator with type II polarization dependence.

In short, if both the isolator 124 and the coupler 110, as well as anyother component added between the erbium-doped fiber 118 and the inputpolarizer to a gyroscope, exhibit a weak type II polarizationdependence, a second depolarizer 302 is not required. However, if one ormore of these components exhibits a strong type II polarization, i.e.,if one polarization is much more strongly attenuated than its orthogonalpolarization, a second depolarizer 302 is required. On the other hand,if either the isolator 124, the coupler 110, or any other componentadded between the erbium-doped fiber 118 and the gyro input polarizerexhibit a type I polarization dependence, a second depolarizer 302 isrequired.

To measure the effectiveness of the second depolarizer 302 in reducingthe polarization dependence of the WDM coupler 110 and the isolator 124,a testbed of the superfluorescent source of FIG. 9A was constructedusing a depolarizer 302 with two high birefringence fiber lengths of 1.5meters and 3 meters, respectively. This testbed is shown in FIG. 9B. Theoptical output of the source was launched into an OSA 137, and thespectrum read by the OSA was captured and analyzed by a computer 140.The OSA 137 was a polarization-dependent optical spectrum analyzer,i.e., the optical spectrum that it produced depends on the polarizationof the light launched into it. Two additional polarization controllers107 and 108 (designated PC₂ and PC₃) are introduced, one on each side ofthe depolarizer 302 (see FIG. 9B). Because the OSA 137 produces aspectrum that depends on the polarization of the input light, if thedepolarizer 302 were not effectively depolarizing the light from the SFS118, the OSA would read a spectrum mean wavelength that depends on theorientation of either polarization controller 107 or 108. However, whenthe respective orientations of the polarization controller 107 and 108are varied (see FIGS. 4C and 4D), the mean wavelength of the SFSspectrum measured by the OSA 137 is found to be within the system noiselimit of 3 ppm. Consequently, the depolarizer 302 effectively reducesthe polarization dependence of the SFS spectrum introduced by the WDMcoupler 110 and the isolator 124.

To further assess the effectiveness of the two depolarizers 300 and 302in reducing variations in the mean wavelength of an SFS due toenvironmental perturbations, the erbium-doped fiber 118 was placed in awater bath 200 at room temperature, as shown in FIG. 10, and recordedthe mean wavelength of the source as a function of time for a period of10 hours. During this period, none of the components in FIG. 10 wasadjusted. The temperature of the bath 200 was unregulated, but it variedby at most ±1° C., and probably by only ±0.5° C., during the course ofthis test. The OSA 137 of FIG. 10 is the polarization-dependentinstrument described earlier. The result of this test is shown in FIG.11 in the form of the mean wavelength of the source plotted versus time.FIG. 11 shows that the mean wavelength of the source exhibits someshort-term variations, of the order of 4 ppm peak to peak, as well aslong-term variations, of the order of 6 ppm peak to peak. The same fibersource without the two depolarizers 300 and 302 exhibited considerablylarger peak-to-peak variations in mean wavelength, measured to be 50 ppmin one 80 minute test. The conclusion is that the use of the twodepolarizers 300 and 302 substantially improves the overall stability ofthe source mean wavelength.

In the embodiment of FIG. 9C, optical output is directed into aninternal polarizing element (i.e., a polarizer, not shown) of a fiberoptic gyroscope 402, in which the optical output passes through thepolarizer before entering the rotation sensing coil (not shown) of thegyroscope. (All of the superfluorescent sources disclosed herein mayadvantageously be used as optical input to a fiber optic gyroscope.)Three depolarizers 300, 302, 304 are shown, and one, two, or all threeof these depolarizers may be used. The depolarizer 300 alone may besufficient if the coupling ratio of the WDM coupler 110, thetransmission of the WDM coupler 110, and the transmission of theisolator 124 are polarization independent. The depolarizer 304depolarizes the pump light in one direction and ASE traveling in theother direction. The depolarizer 302 depolarizes the output signal toremove polarization effects introduced by the WDM coupler 110 and theisolator 124. However, the depolarizer 302 does not correct forpolarization-dependent gain, so that if there is polarization-dependentgain in fiber 118, either the depolarizer 300, the depolarizer 304, orboth depolarizers, must be used in addition to depolarizer 302 tocorrect for this problem. All depolarizers introduce a small but finiteloss. In the limit that the depolarizers 300, 302, and 304 becomelossless and inexpensive, it becomes advantageous to use all three ofthem.

Another method of reducing the pump polarization dependence of thesource mean wavelength is to pump the superfluorescent source with two(or more) linearly polarized pump sources with substantially the samespectrum. As illustrated in FIG. 12A, a first pump source 1300 and asecond pump source 1310 can be multiplexed through a polarization mixer1320, a device that couples a first pump having a first polarizationfrom port 1 to port 3, and a second pump having a second polarizationfrom port 2 to the same port 3, with the two pump polarizations beingperpendicular in port 3. The polarization mixers herein can be, forexample, either a polarizing cube or a polarization-dependent coupler,such as an all fiber polarization dependent coupler or a fiber pigtailedbulk optic polarization coupler. In FIG. 12A, the primed componentsfunction like their analogous components of FIG. 3, except that theprimed components (the output pigtail 104′, the optical coupler 110′,the doped fiber 118′, and the isolator pigtails 120′ and 128′) are allpolarization maintaining. (Alternatively, the output pigtail 104′, theoptical coupler 110′, the doped fiber 118′, and the isolator pigtails120′ and 128′ of FIG. 12A may be non-PM components.)

Embodiments in addition to the embodiment shown in FIG. 12A are nowdescribed which likewise rely on pumping an erbium-doped fiber (EDF)with two orthogonally polarized pump sources. These embodiments areillustrated in FIGS. 12B, 12C, and 12D, corresponding respectively to aforward-pumped configuration, a double-pass configuration, and adouble-pass configuration with an additional Lyot depolarizer.

A forward superfluorescent source is illustrated in FIG. 12B, in whichtwo linearly polarized pump sources 1300, 1310 are optically combinedwith a polarization mixer 1330. The polarization mixer 1330 in FIG. 12Bis shown as a polarization fiber coupler having input fibers 1322, 1324and output fibers 1326, 1328, although a cube beam splitter may be usedas the polarization mixer, for example. The two pump sources 1300, 1310are coupled to the input fibers 1322, 1324, respectively, and deliverthe same power. The input fibers 1322, 1324 are advantageously PM fiber.The two pump sources 1300, 1310 are launched along a differentbirefringence axis of the PM fibers 1322, 1324. The polarization fibercoupler 1330 transmits one polarization (for example, the polarizationin the plane of FIG. 12B), but essentially fully couples the orthogonalpolarization (e.g., the polarization perpendicular to the plane of FIG.12B). Consequently, the output fiber 1326, which is coupled to an erbiumdoped fiber 118, carries pump light having two orthogonal polarizationcomponents of equal power. The output fiber 1326 and the erbium-dopedfiber 118 to which it is optically connected are not necessarily PMfiber. (The output port corresponding to fiber 1328 may or may not beused.) An optical isolator 124 reduces unwanted optical feedback. As thetwo pump signals from pump sources 1300, 1310 travel through the inputfibers 1322, 1324, the states of polarization of the signals remainorthogonal to each other at all points along the input fibers. Thus, theerbium-doped fiber 118 is pumped with two orthogonal pump signals, andthe gain of the erbium-doped fiber 118 is free of polarizationdependence.

For the gain of the erbium-doped fiber 118 to be completely free ofpolarization dependence, the two pump polarizations in the output fiber1326 should carry identical power. Ideally, the polarization mixer 1330completely transmits the pump signal of the pump source 1300 into thefiber 1326 (i.e., this transmission T₁ is unity), and the polarizationmixer 1330 completely couples the pump signal of the pump source 1310into the fiber 1326 (i.e., this transmission T₂ is unity). However, somepolarization mixers exhibit a T₁ that differs (sometimes only slightly)from T₂, for example, due to polarization-dependent loss or a designthat is less than optimum. In this case, if the pump powers launchedinto the fibers 1322 and 1324 are equal, the two pump powers in theoutput fiber 1326 will be slightly different. To correct for thisimbalance, one or both of the pump powers launched into fibers 1322 and1324 must be adjusted accordingly.

Two orthogonally oriented pump sources 1300, 1310 are used in the doublepass SFS configuration illustrated in FIG. 12C. A dichroic reflector1450 is placed at the pump input end of the EDF 118 so that this SFSoperates in a double-pass configuration. The dichroic reflector 1450 isdesigned to have (1) a high reflection coefficient (ideally 100%) over agood fraction (ideally all) of the source's ASE spectral range, and (2)a high transmission coefficient (ideally 100%) at the pump wavelength.Once again, the polarization mixer 1330 is illustrated as being apolarization fiber coupler, although a cube beam splitter can be used.The embodiment of FIG. 12C offers the advantages of a lower threshold, alower pump power requirement, and a shorter required length of theerbium-doped fiber. The dichroic reflector 1450 may be a bulk-opticdevice, a fiber Bragg grating reflector, or it may comprise multipledielectric layers deposited directly onto the pump input end of the EDF118. The pump output end of the EDF 118 in FIGS. 12A, 12B, 12C, and 12D(discussed below) is optically terminated, for example, by polishing orcleaving the end of the fiber 118 at an angle, or splicing it to acoreless fiber. The fiber end may be followed (in the optical pathsense) by an optical isolator 124. As an alternative to the double-passsuperfluorescent source illustrated in FIG. 12C, a dichroic reflector1450 may be placed in the embodiment of FIG. 12A between the WDM coupler110′ and the input pigtail 120′ of the isolator 124, but this wouldnecessitate making the left end of the EDF 118′ the output port andplacing an optical isolator at that end.

The effectiveness of the embodiments shown in FIGS. 12A, 12B, and 12Cmay be improved by placing a depolarizer in the path of the multiplexedpump signals. For example, in the case of FIG. 12A, a depolarizer 1460may be placed between the output pigtail 104′ of the polarization mixer1320 and the WDM coupler 110′. In FIG. 12B, a depolarizer 1460 may beplaced between the polarization mixer 1330 and the EDF 118. In theembodiment of FIG. 12C, a depolarizer 1460 may be placed between thepolarization mixer 1330 and the dichroic reflector 1450, as illustratedin FIG. 12D. The depolarizer 1460 in FIG. 12D (which can, for example,be a Lyot depolarizer either in a bulk-optic form or a fiber form)depolarizes each of the two orthogonal pump signals traveling throughit. The role of the depolarizer 1460 is to distribute the power in eachpump signal evenly onto two orthogonal polarization axes, which may beimportant if the two pump sources 1300, 1310 do not generate the samepump power. If a depolarizer is not used (e.g., FIGS. 12A, 12B, and 12C)and the pump sources 1300 and 1310 generate different power levels,there will be some polarization-dependent gain. In the embodiment shownin FIG. 12D, however, the depolarizer 1460 acts to reduce this residualpolarization dependent gain, and thus to reduce the dependence of themean wavelength of the SFS on the input polarization of the pump sourcesand any difference in power between the two pump sources.

Alternatively, as illustrated in FIG. 13A, the superfluorescent sourcecan be bidirectionally pumped, i.e., pumped from one end with a pumpsource 1410 having a first polarization, and pumped from the other endwith a pump source 1420 having a second polarization orthogonal to thefirst. Output from the pump source 1410 may be advantageously passedthrough an isolator 1430. Likewise, output from the pump source 1420 maybe passed through an isolator (not shown) positioned between the pumpsource 1420 and the WDM coupler 110′. With both pump sources 1410 and1420, care must be taken to use free space or high-birefringentwaveguides to bring the pump light from the pump source to thesuperfluorescent fiber 118′, so that the pump light enters thesuperfluorescent source with the proper polarization. In this method, itis important that for every frequency component, the polarizations ofthe two pump signals remain essentially orthogonal along the length ofthe superfluorescent source. One solution for satisfying thisrequirement is to make the superfluorescent source from a short opticalwaveguide, such as a standard single-mode fiber, a planar-geometry orintegrated optic waveguide, with a high dopant concentration. A secondsolution for satisfying the orthogonality requirement discussed above isto make the EDF from a high-birefringence single-mode fiber, or to use ahigh-birefringence planar-geometry or a high-birefringence integratedoptic waveguide. In this case, one of the pumps is launched along theslow axis of the guide, and the other pump along the fast axis of theguide, either in the same or opposite directions. The high birefringenceguarantees that the two pumps remain orthogonal along the entire lengthof the waveguide. In both cases, the spectra of the two pump sources donot need to be identical. The spectra can in fact be different, providedthat they are such that substantially the same optical gain spectrum iscreated for light propagating along either one or the other of thebirefringence axes of the waveguide.

Even when the fiber 118′ of FIG. 13A is pumped simultaneously inopposite directions with the same power, simulations show that theforward and backward ASE outputs from the fiber 118′ are still slightlypolarized, such that their mean wavelength difference A is not zero. If,for example, pump light from the first pump 1410 (which is linearlypolarized) is launched along the x axis of fiber 118′, and pump light ofequal power from the second pump 1420 (which is also linearly polarized)is launched along the y axis, the ASE exiting to the left in FIG. 13A(ASE₁) is slightly polarized along the x axis, while the ASE exiting tothe right in FIG. 13A (ASE₂) is slightly polarized along the y axis. Thephysical explanation for this phenomenon is that ASE₁ is the sum ofbackward ASE generated by the first pump source 1410 and forward ASEgenerated by the second pump source 1420. Since backward ASE is alwaysmore intense than forward ASE (unless the fiber is very short, in whichcase they are identical), the largest contribution to ASE₁ is backwardASE generated by the first pump source 1410, with ASE₁ being slightlypolarized along x. Conversely, ASE₂ is mostly generated by they-polarized second pump source 1420, with the result that ASE₂ isslightly polarized along the y-axis. Consequently, both ASE₁ and ASE₂have non-zero Δ. As these arguments demonstrate, in order to generate anASE emission with Δ=0, it is not in general sufficient to pumpbidirectionally, and it is advantageous to eliminate the residualasymmetry inherent in the embodiment of FIG. 13A.

One way to eliminate this residual asymmetry is to introduce a smalldifference in the powers of the two pump sources 1410, 1420. That such adifference in the powers of the pump sources 1410, 1420 can eliminatethis asymmetry can be understood from physical considerations asfollows. In the event that one of the pump sources is turned offcompletely, e.g., the second pump source 1420, then ASE, is stillslightly polarized along the x-axis, i.e., its mean wavelength along thex-axis, <λ_(x)>, differs from its mean wavelength along the y-axis,<λ_(y)>. Specifically, <λ_(x)> is slightly shorter than <λ_(y)>, and themean wavelength difference Δ=<λ_(x)>-<λ_(y)> is negative. If, on theother hand, the second pump source 1420 is on while the first pumpsource 1410 is off, then ASE₁ is entirely generated by the second pumpsource, which is polarized along the y-axis, such that ASE₁ is slightlypolarized along the y-axis. In this case, the mean wavelength of ASE₁along the y-axis, <λ_(y)>, is slightly shorter than that along thex-axis <λ_(x)>, and the difference Δ=<λ_(x)>-<λ_(y)> is positive. Inshort, when the power P₁ of the first pump source 1410 is finite and thepower P₂ of the second pump source 1420 is zero, Δ is negative, whereaswhen P₁ is zero and P₂ is finite, Δ is positive. It follows that theremust be a specific combination of powers P₁ and P₂ for which Δ=0.

A method of selecting the appropriate power P₂ (for a given power P₁)that produces Δ=0 is illustrated in FIG. 13B. FIGS. 13B and 13C weregenerated with the fiber amplifier computer code numerical simulatordeveloped by Wagener that is mentioned above. In FIG. 13B, the meanwavelength difference Δ for the output ASE₁ (on the left side of the EDF118′ in FIG. 13A) is plotted as a function of the pump power P₂ launchedinto the EDF by the second pump source 1420, for constant pump power P₁of 30 mW. The mean wavelength difference Δ is expressed in ppm, i.e., Δis normalized to the average of the mean wavelengths of the twopolarizations. The various curves were computed for different lengths ofEDF, namely 2 m, 4 m, 6 m, and 8 m. As predicted from physicalconsiderations above, as the power P₂ is increased from below P₁=30 mWto above 30 mW, the mean wavelength difference Δ goes from negative topositive for all lengths of the EDF 118′. Thus, for each of the fiberlengths modeled there is a finite pump power P₂ that causes Δ to bezero. In all cases, this power is lower than the power P₁ launched bythe first pump source (30 mW). FIG. 13B shows that by properly selectingthe power launched by the second source 1420, the light emitted towardsthe first pump source 1410 can be made fully unpolarized.

In FIG. 13C, the output ASE₂ entering the coupler 110′ is calculated fora P₁ of 30 mW. Once again, the mean wavelength a goes from negative topositive, and for each fiber length modeled, there is a finite pumppower P₂ that causes Δ to be zero, which is higher than the power P₁launched by the first pump source (30 mW). Thus, FIG. 13C shows that byproperly selecting the power launched by the second source 1420, thelight emitted towards the second pump source 1420 can be made fullyunpolarized.

According to another embodiment of the invention, shifts in the meanwavelength of output from an SFS are substantially reduced by fixing thestate of polarization of the optical pump source and usingpolarization-maintaining (PM) fibers throughout the fiber source. Thisdiffers from the prior art (see, for example, U.S. Pat. No. 5,701,318 toDigonnet, et al.) because all the optical components discussed in theinstant application are polarization maintaining, and not just theerbium-doped fiber.

One example of such an embodiment is illustrated in FIG. 14A, in whichthe primed components function much like their analogous unprimedcomponents of FIG. 3, except that the primed components (the outputpigtail 104′, the optical coupler 110′, the doped fiber 118′, and theisolator pigtails 120′ and 128′) are all polarization maintaining. Inthis embodiment, the output from the optical pump source 100 (which ispreferably a laser diode) is launched along one of the fiber axes ofpigtail 104′. Consequently, the pump and the superfluorescent signalremain aligned with those axes regardless of environmental conditions.However, this embodiment requires careful alignment of the fiber axes ofadjacent fiber optic components as those components are splicedtogether. In this embodiment, the two eigenpolarizations at the outputof the SFS carry slightly different spectra, because of polarizationdependent gain, but these two spectra are independent of externalperturbation of the fiber birefringence, so that the spectrum (which maybe input to a gyroscope), as well as the spectrum mean wavelength, arestable.

FIG. 14B illustrates an embodiment in which the doped fiber 118″comprises a single-polarization fiber. Careful alignment of the fiber128′ with respect to an input polarizer (not shown) within the fiberoptic gyroscope 402 is required. Alternatively, an additional polarizer420 may be positioned within the fiber 118′, as indicated in FIG. 14C.The optimum location of the additional polarizer 420 can be calculatedin accordance with U.S. Pat. No. 5,701,318 to M. Digonnet et al., whichis incorporated by reference herein. The single-polarization fiberembodiment and the embodiment of FIG. 14C produce superfluorescentoutputs that are essentially linearly polarized, but with a power thatis nominally the same as an unpolarized configuration. Thus, with theembodiment of FIG. 14C, for example, the power delivered through theinput polarizer of the fiber optic gyroscope 402 is effectively doubled.

In FIGS. 14A, 14B, and 14C, the polarization state of light travelingthroughout the fiber (118′, 118″) is frozen withpolarization-maintaining or single-polarization fiber, and the problemof pump polarization drift that prevails in non-polarization maintainingSFSs is substantially eliminated.

A double-pass configuration which utilizes polarization maintainingcomponents is shown in FIG. 15. In this embodiment, the WDM coupler 110′of FIGS. 14A-C is replaced with a dichroic reflector 1450 which ispositioned between the pump source 1300 and the EDF 118′. The lightemitted by the pump source 1300 is linearly polarized and launched alongeither one of the two birefringence axes of the PM erbium-doped fiber118′. An optical isolator 124 is placed at the output end of theerbium-doped fiber 118′ to prevent reflected light (for example, from anoptical system into which the output from the EDF 118′ is coupled) fromcausing the EDF to oscillate. The isolator 124 is advantageously madewith PM fiber, and its birefringence axes are aligned with thebirefringence axes of the erbium-doped fiber 118′ in order to preservethe polarization of the output light. By eliminating the need for a WDMcoupler, the cost of the device is reduced, and a shorter fiber 118′ anda lower pump power is required than the embodiment of FIG. 14A. When theembodiment of FIG. 15 is used as a light source for a fiber opticgyroscope, its output polarization should be aligned with the polarizerat the input of the gyroscope circuit.

A backward output configuration utilizing PM fibers is shown in FIG. 16.The main difference between this embodiment and the configuration ofFIG. 14A is that the output pigtail 1464 of the PM WDM coupler 110′ iscoupled (e.g., spliced) to the PM erbium doped fiber 118′ so that thebirefringence axes of the PM WDM coupler 110′ are aligned at 45 degreeswith respect to the birefringence axes of the PM erbium-doped fiber.Upon exiting the WDM coupler 110′, the pump light enters the PMerbium-doped fiber 118′. As in the case of FIG. 14A, one end 1470 of thepump output end of the EDF 118′ of FIG. 16 is optically terminated,e.g., by polishing or cleaving the fiber end at an angle or splicing itto a coreless fiber, or by coupling it to an optical isolator (notshown).

Since the pump light is launched with equal power into the birefringenceaxes of the EDF 118′, the state of polarization (SOP) of the pump lightvaries periodically along the EDF 118′, with a period L_(b) that dependson the fiber birefringence but that is typically in the range of a fewmm or less. Similarly, the SOP of every frequency component of the ASEsignal traveling in the fiber 118′ also varies periodically along theEDF 118′, with a period L_(b)′ that is different from L_(b) (primarilybecause the pump and the signal have different wavelengths). Thus, atsome periodic locations along the EDF 118′, the pump light and a givenfrequency component of the ASE signal have parallel (linear or circular)polarizations, while at other, likewise periodic locations along the EDF118′, the pump light and the given frequency component of the ASE signalhave orthogonal (linear or circular) polarizations. If the period isshort enough, namely much shorter than the length of the fiber 118′,this given frequency component will experience gain due to pump lightthat is polarized alternately parallel and orthogonal to this frequencycomponent. Consequently, the variations in gain arising from variationsin the polarization of the pump light along the EDF 118′ are averagedout, and this given frequency component of the ASE signal does notexperience PDG. Since this argument holds for every frequency componentof the broadband ASE signal, the source of FIG. 16 does not experiencePDG, and the device of FIG. 16 emits broadband ASE light having the samemean wavelength for all polarizations.

This principle can be extended to a double-pass source configuration byadding a dichroic reflector 1450 at the pump output end of the EDF 118′.As illustrated in FIG. 17, an advantage of such a configuration over theone of FIG. 16 is that the double-pass feature of FIG. 17 permits alower pump power and a shorter length of erbium-doped fiber 118′. Ifneed be, an optical isolator (not shown) can advantageously be placedbetween the pump source 1300 and the WDM coupler 110′ in order toprevent lasing of the EDF 118′ resulting from optical feedback betweenthe dichroic reflector 1450 and optics in the laser 1300 (e.g., theoutput facet of a semiconductor laser.) Another double-pass embodimentis shown in FIG. 18, in which a dichroic reflector 1450 is placedbetween the pump source 1300 and the EDF 118′, thereby eliminating theneed for a WDM coupler and reducing the cost of the device.

FIG. 19 shows another SFS embodiment whose output has a mean wavelengththat is stable with respect to variations in pump polarization andenvironmental perturbations of the circuit fiber. All of the opticalcomponents of FIG. 19 are advantageously made from PM fiber. Unlike theembodiment of, for example, FIG. 13A, the embodiment shown in FIG. 19utilizes one rather than two pump sources. Linearly polarized light froma pump source 1300 is sent through a first optical isolator 124 a into aWDM polarization coupler 1480, which directs the pump light to one oftwo output ports 1482, 1484. The polarization coupler 1480 operates suchthat (1) a certain fraction of the pump light incident upon the mixer1480 is coupled to the left output port 1482, with this fraction of thepump light having a power P₁ and a polarization that is linear, e.g., inthe plane of FIG. 19; and (2) the remaining pump light is coupled to theright output port 1484, and has a power P₂ and a linear polarizationthat is orthogonal to the light entering the left output port 1482,i.e., perpendicular to the plane of FIG. 19 in this example. Thus, theEDF 118′ is pumped bidirectionally by pump signals that are orthogonallypolarized with respect to each other. The coupling ratio of the mixer1480, i.e., the ratio P₂/(P₁+P₂), is selected so that the ASE outputfrom the right side of the EDF 118′ has a Δ of 0 (i.e., a meanwavelength that does not vary with the polarization of the pump light),in accordance with the methodology discussed in connection with FIGS.13A, B, and C. The polarization mixer 1480 should be such that itcouples nominally 0% of the ASE signal in a polarization dependentmanner. In this case, the ASE output from the left hand side of the EDF118′ will in general have a non-zero Δ, but a second optical isolator124 b prevents this signal from reaching the mixer 1480, which wouldotherwise leak through the mixer 1480 and combine with the ASE outputfrom the right side of the EDF 118′ to produce an undesirably phasesensitive output spectrum having a non-zero Δ.

One advantage of the embodiment of FIG. 19 is that only a single pumpsource 1300 is required. This results in lower cost and avoids a problemthat may arise when two pump sources are used, namely, the pump sourcesmay “age” at different rates. If, in the embodiment of FIG. 13A, forexample, the output powers of the two pump sources 1410 and 1420deteriorate over time at different rates, then Δ will vary over time. Onthe other hand, as the pump source 1300 in the embodiment of FIG. 19ages, both P₁ and P₂ will drop over time, but their ratio will remainunchanged which tends to mitigate any changes in Δ.

A further embodiment of the invention is based on the same principle asFIGS. 16, 17, and 18. The EDF still exhibits a strong birefringence, butthis time the EDF is made of a standard, low-birefringence fiber (i.e.,a non-PM fiber) and the birefringence is induced by bending the EDFaround a mandrel with a small enough diameter. The resulting EDF coilexhibits a linear birefringence with two proper axes, one perpendicularto the plane of the coil and the other one parallel to the plane of thecoil. The polarized pump is launched into the coil with its polarizationsuch that equal power is launched into each of these axes. As in theembodiment of FIG. 16, for example, because the beat length of thisbirefringent coil depends on wavelength, the polarization of the pumpand the polarization of the ASE signal evolve periodically along thefiber, with different periods. Consequently, the ASE signal overlapsperiodically with a pump that is orthogonal to it, then parallel to it,which reduces PDG. For the reduction in PDG to be substantial, the beatlength must be short compared to the strong-signal absorption length ofthe EDF, i.e., the bending radius must be small enough. Anotherembodiment of the same concept is to wrap the fiber in a coil, but alsotwist the fiber on itself.

Although preferred embodiments of the present invention have beendescribed in detail above, it will be understood by those of ordinaryskill in the art that certain obvious modifications and departures fromthe embodiments described herein can be made without departing from thespirit or essential characteristics of the invention. For example, inthe embodiments disclosed herein, it is understood that some or all ofthe optical components that make up the embodiments can be replaced byequivalent integrated optic components performing the same function,including, but not limited to, the polarization-maintaining fibers, thedepolarizers, the fiber couplers, the isolators, and the erbium-dopedfibers. An erbium-doped fiber can be replaced by an integrated opticwaveguide based on silica or other materials, having an appropriatelength and erbium concentration. As another example, the fiber couplersdisclosed herein may be constructed with integrated optic waveguidesusing well-known technology. Care should be taken to design this couplerso that it exhibits the appropriate properties, e.g., Type IIpolarization dependence.

What is claimed is:
 1. A superfluorescent source, comprising: an opticalpump source generating optical radiation that is substantiallyunpolarized; and a solid state laser medium that is pumped by thesubstantially unpolarized optical radiation, said medium havingcharacteristics selected to generate superfluorescence having a fullwidth at half maximum (FWHM) of at least 2 nm and a mean wavelength thatis stable to within 50 ppm against polarization fluctuations in saidsuperfluorescent source.
 2. The superfluorescent source of claim 1,wherein the superfluorescence from said medium is generated by rareearth ions.
 3. The superfluorescent source of claim 1, wherein thesuperfluorescence from said medium is generated by erbium ions.
 4. Thesuperfluorescent source of claim 3, comprising an optical couplingelement that couples the optical radiation to said medium.
 5. Thesuperfluorescent source of claim 4, wherein said optical couplingelement is a wavelength division multiplexing (WDM) coupler.
 6. Thesuperfluorescent source of claim 1, comprising an isolator to reduceoptical feedback.
 7. The superfluorescent source of claim 3, furthercomprising a fiber optic gyroscope, wherein the superfluorescence ofsaid superfluorescent source is input into said gyroscope.
 8. Thesuperfluorescent source of claim 3, wherein said optical pump sourcecomprises: a laser; and a depolarizer.
 9. The superfluorescent source ofclaim 8, wherein said depolarizer is a Lyot depolarizer.
 10. Thesuperfluorescent source of claim 8, wherein said depolarizer includes aPZT modulator.
 11. The superfluorescent source of claim 8, furthercomprising a second depolarizer on the output side of said laser medium.12. The superfluorescent source of claim 1, wherein thesuperfluorescence has a mean wavelength that is stable to within 3 ppmagainst fluctuations in said superfluorescent source.
 13. Thesuperfluorescent source of claim 8, wherein said laser includes a laserdiode.
 14. The superfluorescent source of claim 1, wherein said solidstate medium comprises an optical waveguide.
 15. The superfluorescentsource of claim 1, wherein said solid state medium comprises an opticalfiber.
 16. The superfluorescent source of claim 1, wherein the meanwavelength is stable to within 50 ppm against birefringence changes insaid superfluorescent source.
 17. The superfluorescent source of claim1, wherein the mean wavelength is stable to within 50 ppm againstpolarization changes of the optical radiation from said optical pumpsource.
 18. The superfluorescent source of claim 1, said optical pumpsource comprising a plurality of pumps, said plurality of pumps havingoutputs with polarizations selected such that the optical radiation fromsaid optical pump source is substantially unpolarized.
 19. Thesuperfluorescent source of claim 18, wherein said plurality of pumpsincludes two pumps whose respective optical outputs are combined so thatthe polarizations of the outputs are orthogonal to each other.
 20. Asuperfluorescent source, comprising: an optical pump source generatingoptical radiation that is substantially unpolarized; and a solid statelaser medium that is pumped by the substantially unpolarized opticalradiation, said medium having characteristics selected to generatesuperfluorescence having a full width at half maximum (FWHM) of at least2 nm and a mean wavelength that is stable to within 50 ppm againstpolarization changes in said source that range over the Poincairesphere.
 21. A superfluorescent source, comprising: an optical pumpsource generating optical radiation that is substantially unpolarized,said optical pump source including: a plurality of pumps that generaterespective optical outputs; a polarization mixer that receives therespective optical outputs from said plurality of pumps and generatesoptical output, wherein the respective optical outputs from saidplurality of pumps have polarizations selected such that optical outputfrom said mixer is substantially unpolarized; and a depolarizer thatreceives the optical output from said polarization mixer; and a solidstate laser medium that is pumped by the output from said depolarizer,said medium having characteristics selected to generatesuperfluorescence having a full width at half maximum (FWHM) of at least2 nm and a mean wavelength that is stable against polarizationfluctuations in said superfluorescent source.
 22. The superfluorescentsource of 21, further comprising a dichroic reflector between saiddepolarizer and said medium.
 23. The superfluorescent source of claim21, wherein the mean wavelength is stable to within 500 ppm againstpolarization fluctuations in said superfluorescent source.
 24. Thesuperfluorescent source of claim 21, wherein the mean wavelength isstable to within 100 ppm against polarization fluctuations in saidsuperfluorescent source.
 25. The superfluorescent source of claim 21,wherein the mean wavelength is stable to within 50 ppm againstpolarization fluctuations in said superfluorescent source.
 26. Thesuperfluorescent source of claim 21, wherein the mean wavelength isstable to within 3 ppm against polarization fluctuations in saidsuperfluorescent source.
 27. The superfluorescent source of claim 21,wherein said plurality of pumps includes two pumps having respectiveoptical outputs combined so that the polarizations of the outputs areorthogonal to each other.
 28. The superfluorescent source of claim 21,wherein the superfluorescence from said medium is generated by erbiumions.
 29. The superfluorescent source of claim 21, wherein thesuperfluorescence from said laser medium is generated by rare earthions.
 30. The superfluorescent source of claim 21, comprising anisolator to reduce optical feedback.
 31. The superfluorescent source ofclaim 21, further comprising a fiber optic gyroscope, wherein thesuperfluorescence of said superfluorescent source is input into saidgyroscope.
 32. The superfluorescent source of claim 21, wherein saidplurality of pumps comprise lasers.
 33. The superfluorescent source ofclaim 21, wherein said depolarizer is a Lyot depolarizer.
 34. Thesuperfluorescent source of claim 21, wherein said depolarizer includes aPZT modulator.
 35. The superfluorescent source of claim 21, wherein saidsolid state medium comprises an optical waveguide.
 36. Thesuperfluorescent source of claim 21, wherein said solid state mediumcomprises an optical fiber.
 37. The superfluorescent source of claim 21,wherein the mean wavelength is stable to within 500 ppm againstbirefringence changes in said superfluorescent source.
 38. Thesuperfluorescent source of claim 21, wherein the mean wavelength isstable to within 500 ppm against polarization changes of the opticalradiation from said optical pump source.
 39. The superfluorescent sourceof claim 21, further comprising a dichroic reflector between saiddepolarizer and said medium, said reflector reflecting backwardtraveling optical radiation from said medium back through said medium.40. A method of generating superfluorescence, comprising: providing aplurality of optical pumps, the pumps having respective optical outputswith different polarizations; directing the respective optical outputsthrough a polarization mixer that produces optical output, wherein thedifferent polarizations are selected so that the optical output from themixer is substantially unpolarized; depolarizing the output from themixer; injecting the depolarized output into a solid state laser medium;and producing superfluorescence from the medium that has a meanwavelength that is stable against polarization fluctuations in thesuperfluorescent source.
 41. The method of claim 40, wherein thewavelength is stable to within 500 ppm against polarization fluctuationsin said superfluorescent source.
 42. The method of claim 40, wherein thesuperfluorescence has a full width at half maximum (FWHM) of at least 2nm.
 43. The method of claim 40, wherein said depolarizing includesdirecting the output from the mixer through a depolarizer.
 44. Themethod of claim 40, wherein the plurality of optical pumps includes twopumps having respective optical outputs whose polarizations areorthogonal to each other.
 45. The method of claim 40, wherein the solidstate medium includes rare earth ions.